According to the US National Highway Traffic Safety Administration, traffic crashes were the leading cause of death for the age group 4 through 34 in 2003. It is known that traffic fatalities increase dramatically with an increase in vehicle speed. Work in the area of Intelligent Transportation Systems envisions a wireless communications infrastructure encompassing both fixed roadside units and mobile vehicular units mounted in commercial and private motor vehicles. One application of such a system is to share real-time safety information among vehicles in a local area. For example, a basic safety message that is intended to be broadcast repeatedly to surrounding vehicles contains data elements such as, position, motion, control, and vehicle size.
These messages and others are transmitted on a wireless channel with limited capacity. If more messages are being generated than can be accommodated by the communications channel, messages will be delayed and possibly lost, negatively affecting the safety of the vehicles and their occupants.
Given the limited spatial range of such wireless systems in a nominal highway situation, a vehicle may be in range of a dozen or so other vehicles at a time. Safety messages broadcast at one message per second per vehicle would impose a light load on the communications channel. If the traffic slows to a “bumper-to-bumper” density, however, there may be hundreds of vehicles within range. In this scenario, the message volume may overwhelm the communications system, reducing its ability to serve its purpose of providing timely delivery of critical information. Since the critical information (e.g., location) associated with a slower moving vehicle is less dynamic than that of a fast-moving vehicle, it is not necessary to update the slower vehicles' information as frequently. Likewise, under otherwise benign conditions, reporting rate can be lessened. Conversely, under adverse conditions, such as precipitation or road damage, traffic safety will benefit from more frequent updates.
U.S. Pat. Nos. 6,240,294 and 6,600,927 address periodic position reports transmitted by mobile military vehicles. They describe two reporting modes: a periodic (PER) mode where position updates are sent at a fixed interval (e.g., every two minutes), and a movement (MOV) mode where position updates are issued when the unit has moved a pre-defined distance (e.g., 300 meters) from the location of its last report. The system described in U.S. Pat. No. 6,600,927 also adjusts the time and distance thresholds based on a measurement of network loading, which is defined as a function of channel idle time. The primary objective of this system is to track the coarse location of the reporting military units.
For example, FIG. 6 shows a vehicle 60, in a MOV mode that sends a first position report. It then moves over an extensive and complex course 61. However, the vehicle 60 does not send a position update as long as it does not move more than a predefined distance from its previously reported position and thus its net movement never exceeds the distance criterion (indicated by radius 62), so it never sends a subsequent update. Furthermore, unicast addressing can provide reliable message delivery through feedback from the recipient, for example, via an ARQ (automatic repeat-request) protocol. However, unicast is not efficient for messages sent to many recipients, since a separate message would need to be sent to each recipients and thus consuming unnecessary channel capacity. Unicast also is not effective for messages sent to unknown recipients, since the address of a nearby desired recipient may not be known.
Repeated transmissions, which means sending a broadcast message more than once, generally increases the chance that at least one copy will be received. However, the disadvantage is unnecessary increase in channel loading.
Similarly, sending a message at higher transmission power increases the chance that the message will be received. However, the higher power negatively impacts the performance of other nearby mobile units by increasing their received interference power level and thus decreasing their ability to receive other traffic.
Alternatively, the sender can transmit its messages in a more robust form, which is more likely to be received. For example, a more powerful forward error correcting (FEC) code (e.g., rate ½ vs. rate ¼); or a more robust modulation rate (e.g., a binary code vs. an 8-level code) could be used. The disadvantage with this method is that the more robust messages are of longer duration and therefore consume more channel capacity, when a less robust, more efficient message may have been delivered with no problem.
Therefore, there is a need for an intelligent vehicle that is capable of broadcasting repeated safety messages at a high albeit variable, environment dependent rate, so that nearby vehicles can quickly adapt to its presence and its movements. The receipt of a timely safety message potentially allows the recipient to avoid collision with the reporting vehicle, for example, by moving out of its path.